I was walking through a park this afternoon and observed a few birds having fun flying around and it got me thinking why would natural selection favour birds with flesh at all if it hinders their flight.

We started first with single-cell and micro-organisms flourishing the ponds and then they evolved into massive dinosaurs. It therefore seems, there are some unique reasons for growing to such a large size.

However, I thought it's counter intuitive because the large body slows down locomotion, requires lots of food and energy upkeep, is a big flesh target for predators, makes hiding ineffective.

Although it is obvious that the flesh protects the internal organs, but it seems like a chicken and egg problem because without a massive body, perhaps then there is no need for the internal organs.

On a different perspective, perhaps those animals that evolved without flesh are called plants.

Situations where large body could help (some are estimations, some are known): birds in winter - large birds can go without food for longer, larger shelled turtles are harder for sharks to attack - they don't fit in to the sharks mouth so easily so less likely to be crushed, blue whales are large - the sea is cold and large animals radiate heat less rapidly.
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GriffinEvoFeb 18 '13 at 9:11

3 Answers
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Well first of all I don't know if natural selection favors larger animals - most of the living things in the world are single celled.

Still there are advantages to being larger. I don't know if I can list them all! In no particular order:

1) Living things that are large are more metabolically efficient. The amount of food required goes as a 3/4 power of the size of the animal. Thus a cat 100 times the mass of a mouse requires only 31 times more food. I don't believe that it is not known how this works.

2) r vs K selection: larger animals have greater individual "value". Smaller animals tend to live by r-selection - they are cheap and reproduce in large amounts, then lead short and highly competitive lives where most offspring die without reproducing. Larger animal tend to live longer and reproduce offspring with a greater chance of life. In the extreme case you can compare a cockaroach to an elephant here. This is not a hard and fas rule, but cheap offspring do tend to be small.

I seem to remember that there are lots of other surveys done and body size correlates to life span and any number of other attributes, which I will try to add after I get some sleep unless someone slips in and lists them all.

3) Cope's rule - an observation as much as a rule that size must convey some advantge because most animals become larger the longer they are around. Not always true, but there is a trend that the longer evolution works, the more likely a large animal will show up. This might be because as niches for smaller animals are filled, niches for larger animals become more accessible?

Unless you're living on an island! If you're reasonably large but got stuck on an island for a long time, you will get smaller. This phenomenon (which also goes the other way - if you're small you tend to get bigger if stuck on an island) is called insular dwarfism: http://en.wikipedia.org/wiki/Island_dwarfism